Wake dynamics and fluid forces of turning maneuvers in sunfish

While experimental analyses of steady rectilinear locomotion in fishes are common, unsteady movement involving time-dependent variation in heading, sp eed and acceleration probably accounts for the greatest portion of the loco motor time budget. Turning maneuvers, in particular, are key elements of th e unsteady locomotor repertoire of fishes and, by many species, are accompl ished by generating asymmetrical forces with the pectoral fins. The develop ment of such left-right asymmetries in force production is a critical and a s yet unstudied aspect of aquatic locomotor dynamics. In this paper, we mea sure the fluid forces exerted by the left and right pectoral fins of bluegi ll sunfish (Lepomis macrochirus) during turning using digital particle imag e velocimetry (DPIV), DPIV allowed quantification of water velocity fields, and hence momentum, in the wake of the pectoral fins as sunfish executed t urns; forces exerted during turning were compared with those generated by t he immediately preceding fin beats during steady swimming. Sunfish generate the forces required for turning by modulating two variables: wake momentum and pectoral fin stroke timing. Fins on opposite sides of the fish play fu nctionally distinct roles during turning maneuvers. The fin nearer the stim ulus inducing the turn (i.e. the strong side fin) generates a laterally ori ented vortex ring with a strong central jet whose associated lateral force is four times greater than that produced during steady swimming. Little pos terior (thrust) force is generated by the strong-side fin, and this fin the refore acts to rotate the body away from the source of the stimulus. The co ntralateral (weak-side) fin generates a posteriorly oriented vortex ring wi th a thrust force nine times that produced by the fin during steady swimmin g. Minimal lateral force is exerted by the weak-side fin, and this fin ther efore acts primarily to translate the body linearly away from the stimulus. Turning with the paired fins is not simply steady swimming performed unila terally. Instead, turning involves asymmetrical hn movements and fluid forc es that are distinct in both direction and magnitude from those used to swi m forward at constant speed. These data reflect the plasticity of the teleo st pectoral fin in performing a wide range of steady and unsteady locomotor tasks.